Thermoacoustic devices are structures that are configured to produce acoustic waves in the presence of a temperature differential or, conversely, to produce a temperature differential in the presence of acoustic waves. The earliest recorded instance of the conversion of heat to sound is the Kibitsunokama, an instrument used in historical Japanese shrine rituals. This instrument was mentioned in a Buddhist monk's diary written in 1568, and was described in a story published in 1776. The first record in the scientific literature of the thermoacoustic generation of sound was an experiment by Byron Higgins, in 1777, in which acoustic oscillations in a large pipe were excited by suitable placement of a hydrogen flame inside the pipe.
The Rijke tube, an early extension of Higgins' work, is well known to modern acousticians as a dramatic lecture demonstration. Higgins' research eventually evolved into the modern science of pulse combustion, whose applications included the German V-1 rocket (the “buzz bomb”) used in World War II and the residential pulse-combustion furnace introduced by Lennox, Inc. in 1982.
The Sondhauss tube is the earliest thermoacoustic engine that is a direct antecedent of a thermoacoustic engine of the type used in the present invention. Nearly two hundred years ago, glassblowers noticed that when a hot glass bulb was attached to a cool glass tubular stem, the stem tip sometimes emitted sound. Sondhauss quantitatively investigated the relation between the pitch of the sound and the dimensions of the apparatus.
John W. Strutt (Lord Rayleigh) was the first to provide a qualitative explanation of the process that converted heat to sound in the Sondhauss tube in 1896:
“If heat be given to the air at the moment of greatest condensation, or be taken from it at the moment of greatest rarefaction, the vibration is encouraged.”
Although Rayleigh's qualitative understanding was correct, it was not until nearly a century later that Nikolas Rott published a series of papers which created a detailed theoretical framework that could produce a unified quantitative description of thermoacoustic phenomena and explicitly calculate the behavior of the Sondhauss tube or Taconis tube.
The efficient production of standing sound waves in sealed resonators by thermoacoustic processes started development at the Los Alamos National Laboratory in the early 1980s. By 1988, the field of thermoacoustic energy conversion had advanced to the point where experimentalists were contemplating thermoacoustic engine designs that could be competitive with other traditional heat engine technologies. One of the best-documented standing-wave thermoacoustic engines was described by G. W. Swift.
A standing-wave thermoacoustic engine of the type fabricated and analyzed by G. W. Swift in 1992 includes a porous medium, known as the “stack”, along which the heat flows from a electrically-heated hot heat exchanger to a water-cooled (exhaust) cold heat exchanger. The “thermal core” (i.e., the stack and the two heat exchangers) is contained within a rigid-walled cylindrical pressure vessel that acts as the standing-wave acoustic resonator oscillating in its fundamental plane-wave mode (i.e., the resonator's length is approximately one-half of the acoustic wavelength). The hot end of the resonator is surrounded by thermal insulation and the ambient-temperature end is connected to a variable flow resistor (i.e., needle valve) and a tank that forms an adjustable acoustic load on the engine.